A known type of onboard weighing system for trucks comprises a transducer mounted to a beam that flexes as the weight of the load carried by the truck changes. The sensor measures the degree to which the beam bends, and provides a readout indicating the load carried and/or the gross weight of the vehicle.
Transducers used in onboard weighing systems have included linear, variable differential transformers (LVDTs) and strain gauges. However, prior weighing systems that rely on these transducers are plagued by a number of problems, in particular by temperature drift and by nonlinearity of the transducer output with applied force.
In applications other than truck weighing systems, optical techniques have been proposed on a number of occasions for measuring the stress or flexure of a member under an applied force. An example of an optical stress sensor is U.S. Pat. No. 3,229,511. In the system proposed in that patent, the stress in a wing spar is measured by a system that includes a light source mounted at a first position on the wing spar, and two photosensitive elements mounted at a second position along the wing spar. This system is initially adjusted such that an equal amount of light from the source falls on both photosensitive elements. Subsequently, when the spar is flexed, movement of the beam with respect to the photosensors causes a change in the differential output of the photosensors, which change is measured and converted to a force. Although the described optical system has certain advantages over systems using strain gauges and LVDTs, it remains subject to temperature drift and nonlinearity problems.